1. Field of the Invention
The invention relates to methods of making reshaped flat glass-ceramic parts or articles by forced reshaping of a corresponding green glass part during the ceramicizing process. The reshaping can be partial or full surface. The invention also relates to an apparatus for performing the method and to the glass-ceramic articles made by the method.
2. Related Art
Glass-ceramic parts or articles in the form of glass-ceramic plates are typically used to provide cooking surfaces in modern cooking ranges and as viewing windowpanes for oven, range and fireplace.
For many years cooking ranges with glass-ceramic cooking surfaces and/or covers have been known in the prior art. The cooking ranges with the glass-ceramic cooking surfaces have been marketed because of their specific properties. They have long-lasting resistance to high temperature differences, surface strength and friendliness to cleaning materials, in addition to their desirable aesthetic properties. Typically the glass-ceramic cooking surfaces are used for heating cooking zones and the like with electric cooking elements and gas burners, including both atmospheric gas burners set in the cooking surfaces, and also with radiant burners under the cooking surface. In more recent times also more and more inductively heated cooking devices have been marketed.
It is also possible to prepare food directly on a glass-ceramic cooking surface, e.g. by grilling, besides using these glass-ceramic cooking surfaces for indirect food preparation, i.e. for heating of food in a cooking vessel or container. The glass-ceramic plates for the above-mentioned cooking surfaces are usually made in a continuous melt process, in which a starting glass is first rolled to its nominal thickness and is then cut to the appropriate cooking surface size. In this state the starting material is called green glass or precursor glass. In a second manufacturing step additional edge working occurs, the drilling and decorating, and subsequently thermal treatment, namely the ceramicizing. These processing steps produce the glass-ceramic with its characteristic properties (high temperature resistance and low thermal expansion).
The cooking ranges currently marketed with glass-ceramic plates providing cooking surfaces (in gas cooking ranges with atmospheric burners a cover can be provided) typically have continuous plane surfaces. However many special cooking unit embodiments can only be made with considerable extra expenses or effort or for limited applications. A series of proposals has been made for improvements in cooking surfaces, especially by partial reshaping. Glass-ceramic plates, especially with partially reshaped regions, thus attain ever-stronger significance on the market.
Edges of holes for receiving open, i.e. atmospheric, gas burners in cooking surfaces typically pose a problem. This sort of cooking range typically must be protected with an additional sealing collar around the edge of each hole for overflowing cooking materials or cleaning liquids. It has also been known to bend up the edges of the holes in an expensive bending process as a substitute for the sealing collars. That means that a reshaped raised region is provided around the edge of each hole, as e.g. described in DE 198 13 691 C1.
Furthermore it has been suggested in DE 43 33 334 C2 to fold or bend the region around the hole for the atmospheric gas burner up from the main plane of the cooking surface to form a truncated conical, spherical or circular conical section or to from a prism, tetrahedron, pyramid and/or other section. An arrangement of this sort has, among others, the advantage that scarcely any further measures are required for sealing of the gas burner from the cooking surface (e.g. as a protection against overflowing cooking materials) and also the cleaning of the cooking surface at the transition region from the cooking surface to burner is essentially simplified.
Further desirable shaping of glass-ceramic cooking surfaces results, for example, from the need to fix the cooking grates over the gas burners and to protect switch surfaces from overflowing cooking material or from unintended impacts due to sliding of pots and other cooking vessels according to DE 199 06 911 C1.
Also shaping and reshaping can have functional and aesthetic advantages for viewing windowpanes used in ovens, ranges and fireplaces. For this purpose these panes are already reshaped currently by sinking into a shape under their own weight during a ceramicizing process or before that in a green glass condition by bending or folding or vacuum deep drawing with rapid heating by means of a gas burner or infrared heating. A series of methods for making partial-surface or full-surface glass-ceramic plates are known, of which two typical methods are described next.
WO 97/00407 (=EP 0 834 044 B1) describes a method of reshaping a glass-ceramic plate, especially providing a glass-ceramic cooking surface. For this purpose a glass plate is made in a plastic state with a width, which is larger than that of the cooking surface to be formed, and which has a viscosity of 1000 to 5000 Pas, with the help of a melted precursor (green glass) mass for the glass ceramic by rolling the previously melted glass between rollers. Then the pasty glass sheet or disk is conveyed to a surface of a mold, which is equipped with vacuum devices, so that this glass disk covers the above-described shaping surface, which contains a number of impressions of bulges. The glass disk is then pulled against the surface of the mold by the suction or vacuum produced by a suction apparatus. The parts of the glass disk, which protrude through the surface of the mold, are cut away so that a shaped disk of the green glass is obtained. Subsequently the shaped glass plate or disk is subjected to a heat treatment for ceramicizing it and is converted to a shaped glass-ceramic plate.
This known manufacturing process has the disadvantage that the melt vessel, the rollers and the shaping parts can only be used for this shaping process. The special and very expensive mold permits economical production of only a very large number of individual products.
An additional disadvantage is the visible damage of the plate underside, i.e. the surface in contact with the mold, produced by the method. This surface is covered by a leathery structure formed by shrinkage and flow waves, depending on the existing temperature conditions after the rolling process. This not only interferes with the usually desired aesthetics of the cooking surface, but it also causes strength reductions.
EP 0 879 797 describes a method for introduction of raised portions and depressions in a glass-ceramic cooking surface, which comprises a partial sinking of a bar or cylindrical disk supported on a partially underlying supporting mold during the ceramicizing process, but only because of the action of gravity. A reshaping process occurring because of the force of gravity alone is also disclosed in DE 101 02 576 A1.
In this known reshaping method by sinking due to gravity during ceramicizing a partial reshaping is possible, but only with very great limitations. Only very “soft” reshaping with very large tapering radii can occur. Small local borders, like for example are required for control functions or centering mushrooms for pot support fixtures, cannot be produced with this method. Because of the large tapering radii it is not possible to use this method for electric cooking surfaces, since the planarity of the cooking zones is negatively influenced, which could lead to a reduction in the performance of the cooking apparatus.
DE 100 47 576 A1 discloses a method for reshaping a “glass-ceramic blank”, i.e. a green glass body, during its ceramicizing by a forced reshaping. The forced reshaping can occur by vacuum sinking, by means of a pressing tool or by blowing pressurized air into the respective hollow mold. The green glass body is placed in an IR radiating body equipped with an IR radiator and is heated there to the required temperatures and reshaped. This reference describes a discontinuous process, in which the individual treatments of the green glass body are performed in separate processing steps. The plane green glass plate is put in an IR radiating chamber as a single piece. After that the radiating chamber is heated by means of the IR radiator, the reshaping takes place, and subsequently ceramicizing is performed. Then the process concludes by cooling down the radiating chamber and removing the ceramic plate. This process is comparatively uneconomical because of the discontinuous individual treatment of the green glass plate to be ceramicized.